The 5-HT transporter catalyses the neuronal reuptake of 5-HT from the synapse and it is an important mechanism in the regulation of serotonergic neurotransmission. 5-HT neurons appear to be involved in the neurodegenerative and behaviorally reinforcing effects of drugs of abuse such as cocaine and the amphetamines. This conclusion is supported by observations that "classical" 5-HT uptake inhibitors (e.g. fluoxetine, paroxetine), which selectively bind to and inhibit 5-HT transporter activity, can antagonize the reinforcing effects of cocaine and amphetamine and the neurodegenerative actions of substituted amphetamines. These results are interesting in light of the fact that both cocaine and the amphetamines also bind directly to the 5-HT transporter and inhibit 5-HT uptake. Obviously the nature of the interaction of these drugs of abuse is different from classical 5-HT uptake inhibitors such as fluoxetine. The long term objective of this proposal is to delineate the sites and mechanisms through which cocaine and the amphetamines alter 5-HT transporter function. Such information could lead to the development of treatments to mitigate the neurodegenerative and rewarding effects of these drugs. To achieve this long term objective, the present proposal will investigate the molecular pharmacology of ligand binding to, and transport by, the 5-HT transporter. Specifically, the mechanisms through which cocaine and the amphetamines alter 5-HT uptake and efflux and 3H-paroxetine binding to the 5-HT transporter will be studied using isolated nerve terminals and nerve terminal membrane preparations. In addition the 5-HT transporter will be purified in order to study its biochemical characteristics and functional organization, and how cocaine and amphetamine alter these properties. Finally, the cellular regulation of 5-HT transporter activity by protein phosphorylation will be studied to determine how cocaine and the amphetamines can affect this regulation.